: At birth, pulmonary blood flow increases 8-10 fold and pulmonary arterial (PA) pressure declines steadily over the first several hours of life. When physiologic stimuli do not lead to lower pressure, vasoconstriction remains, causing persistent pulmonary hypertension of the newborn(PPHN). PPHN is characterized by increased pulmonary vascular tone and reactivity, decreased pulmonary blood flow, and severe central hypoxemia. Recent observations suggest that Endothelin-1(ET-1) activity plays a critical role in modulating perinatal pulmonary vascular tone. Our preliminary data demonstrate that normoxic fetal pulmonary artery smooth muscle cells(FPASMC) respond to ET-1 by increasing cytosolic calcium concentrations ([Ca2+]I), suggesting that ET-1 may modulate the contractile state of the pulmonary vasculature in the fetus and newborn. Whether ET-1 interacts with the oxygen tension environment of the normal fetal lung to maintain elevated pulmonary vascular tone is unknown. Evidence that ET-1 increases fetal pulmonary vascular smooth muscle cell cytosolic calcium, as proposed in this application, will provide a novel and specific target for a paradigm shift in the current approach to PPHN, a disease for which neither prevention nor cure is currently available. This proposal tests the working HYPOTHESIS that ET-1 directly increases [Ca2+]I in FPASMC. The specific aims are to test the following hypotheses: Aim 1: ET-1 increases FPASMC [Ca2+]I through release of inositol triphosphate(IP3) sensitive calcium(Ca2+]i through release of inositol triphosphate(IP3) sensitive calcium(Ca2+) stores. Aim 2: The low oxygen environment of the fetal pulmonary vasculature potentates the ET-1 induced increase in [Ca2+]i. We will use Ca2+ fluorescence imaging methods to measure changes in normoxic and hypoxic FPASMC [Ca2+]i in response to ET-1. We will then identify the subcellular mechanisms whereby ET-1 modulates FPASMC cytosolic Ca2+.